The present invention relates to a bearing for a shaft rotating in a turbine engine. It is also aimed at a turbine engine equipped with such a bearing arrangement.
In a turbine engine, the fan is intended to accelerate the air which enters an external gas flow path and joins the normal gas flow path occupied by the compressors, the combustion chamber and, after the latter, the turbines, in order to increase the gas bypass ratio.
In the event of a blade of the fan breaking, a significant unbalance is produced on the rotating shaft, which supports the fan, and consequently significant loads are transmitted to the surrounding fixed structures, entailing the risk of impairing them rapidly. It is therefore necessary to protect these surrounding structures against damage caused by a propagation of significant forces from the rotating shaft.
It could be contemplated to reinforce the surrounding structures to prevent them from being damaged, but that would result in these structures being made heavier, a situation which is at odds with the current objectives of manufacturers.
Generally, the rotating shaft is supported by a first bearing downstream of the fan and by a second bearing downstream of the first bearing. The term “downstream” is considered in the direction of gas flow.
It is known practice to use a decoupler device generally consisting of one or more fusible elements which are designed to yield rapidly to prevent the transmission of significant forces from the rotating shaft to the fixed structures during an exceptional event such as a fan blade breaking. These fusible elements generally form part of a support of the first bearing. By virtue of the decoupling, the rotating shaft can be kept rotating about its axis, with a certain radial flexibility. In other words, it continues to rotate about its axis of inertia which is offset radially with respect to the geometric axis of the engine. After shutting down the engine and slowing down the rotor, the rotational speed stabilizes at a lower value which corresponds to the speed of advance of the aircraft until it returns to the ground.
The rotating shaft forms part of a low-pressure shaft line. Generally, the axial travel of the rotating shaft is limited by an axial stop which is situated at the first bearing.
Configurations are also encountered in which the axial stop is situated at the second bearing. This arrangement makes it possible for the first bearing used to be a roller bearing, which is well adapted to the radial loads experienced in the event of a blade breaking. In normal operation, the takeup of axial forces at the second bearing thus occurs with an advantageous saving in mass. Furthermore, after decoupling, the axial stop function is maintained at the second bearing.
However, such an arrangement, comprising a roller bearing as first bearing, has certain drawbacks. Specifically, after decoupling at the first bearing, the rotation of the rotating shaft continues in a disturbed manner, thus leading to the shaft bending at the second bearing. The second bearing is thus subjected to a significant misalignment which must be accommodated by the rolling elements. This disruptive loading at the second bearing is then transmitted to the surrounding fixed structures. Consequently, these surrounding fixed structures are at risk of being damaged. Furthermore, there is a risk of the bearing cage breaking, resulting in the disappearance of the bearing function at this point.
Document U.S. Pat. No. 6,491,497 describes an arrangement for a second bearing that strives to overcome these drawbacks. This arrangement for a second bearing comprises a pivoting element which comprises a spherical face and which is fastened to the second bearing by means of retention members. These retention members are capable of withstanding the loads of a normal operation, but they break when they are subjected to a misalignment or predetermined moment. These retention members constitute secondary fusible elements which complement the primary fusible elements situated at the support for the first bearing. The breaking of the retention members allows a slight angular displacement between the rotating shaft and the support for the second bearing. Such an arrangement for a second bearing has the beneficial effect of limiting the torque transmitted to the bearing support by the second bearing. However, the radial force passes primarily through the pivoting element. As a result, the force required to break the fusible retention members increases with the applied force and depends on the coefficient of friction, a situation which might lead to an unwanted breaking of the support of the second bearing.